A simple and low-cost optical sensor, based on porous silicon nanotechnology, has been used to detect and quantify the presence of pesticides in water and humic acid solutions. A well defined optical signal variation is registered even at low concentration as 1 ppm. The phenomenon can be ascribed to the capillary infiltration of liquid into the pores, which makes possible to detect changes in the refractive index of the liquid solution up to 0.0002.